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Author Topic: Preamp thoughts  (Read 12126 times)

R.G.

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Preamp thoughts
« on: April 05, 2006, 07:23:43 PM »
I have some thoughts on the design of a preamp. Some of these have been stated here, some haven't.

1. Use a high input impedance. 1M is a minimum.
2. Protect your inputs. What happens if someone hooks up 120Vac from the wall socket to the input? The common reply to this is "No one would be stupid enough to do that." Second reply "Well, they deserve what they get if they do." The correct response is "Cool! How can we do that?" because someone can, and if they can, at some point they will. Or they'll hook up the output of their 1000W car audio amp to it; same thing. Protect your inputs.
3. Know what you're doing with feedback - and without feedback. Feedback is a great tool, but it's often misapplied. High open loop feedback gain stages clip sharp as a razor when they clip. So either use high feedback stages where you *want* razor sharp clipping or where it will (almost) never clip. Use no-feedback or low-feedback stages where you want to hear the device itself. In general, soft clipping is the characteristic of a single device, or a high feedback stage emulating a response curve and never clipping.
4. Know what your power amp wants to see in terms of inputs. Give it that. It is rare for a solid state power amp to clip gracefully. Remember all those quips about getting a pig to sing?

joecool85

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Re: Preamp thoughts
« Reply #1 on: April 05, 2006, 07:28:40 PM »
First off, welcome to the board RG!  Secondly, those are all great ideas!  I'd like to see our preamp(s) use those features.  How would you protect your inputs from stuff like that?
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RDV

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Re: Preamp thoughts
« Reply #2 on: April 05, 2006, 07:31:22 PM »
Thank you kind sir for accepting my invitation. If you could check in and give an opinion here and there about what's going on here we would be most appreciative.

Regards

RDV

joecool85

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Re: Preamp thoughts
« Reply #3 on: April 05, 2006, 07:32:56 PM »
Ah, RDV got you in here  :P
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R.G.

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Re: Preamp thoughts
« Reply #4 on: April 05, 2006, 07:34:48 PM »
Yeah, I'll toss in a thing or two as appropriate. Rick knows a lot of my philosophy already.

joecool85

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Re: Preamp thoughts
« Reply #5 on: April 05, 2006, 07:39:39 PM »
So do I  ;D  I spent countless hours reading chipamp threads on diystompboxes a few months ago before I built my amp.
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RDV

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Re: Preamp thoughts
« Reply #6 on: April 05, 2006, 08:05:24 PM »
I'm building yet another LM3886 amp to permanently mount in my old Peavey 210 Chorus(minus the guts) cabinet which I'm going to run @ 16 ohms(at least for now. It's going to be a platform for trying out SS preamp designs. I didn't want to disturb the Thomas/Vox version which I really like. I've got to come up with a decent chassis which I may have to actually buy this time. :grr I like freebies but I kinda want to make this a little more mobile.

My idea for the preamp is to have high input impedance and come up with a way to lessen the inherent razor sharp clipping R.G. refers to(which I've experienced 1st hand in some of my other designs. I was thinking that a moderate amount of FB gain in an opamp tempured by stacked/back to back diodes(a bunch of them) arranged in such a fashion as to act somewhat like the limiter in the Thomas/Vox preamp but do it up front. I don't want to do this for distortion at all and when it does distort I want it to be as soft as possible and EQ'd to a guitarish style. Then I want to bring the actual level of the signal up from there with subsequant stages resulting in a high headroom sound but having a somewhat soft, tubish hue when pushed hard. I'd like to be able to accomplish this without gyrator stages but I'm not sure I can avoid these because they do such a good job at shaping tone radically.

I hope this doesn't come across as too "pie in the skyish" but it's what I want to try to do and it may take some time.

RDV

joecool85

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Re: Preamp thoughts
« Reply #7 on: April 05, 2006, 08:56:32 PM »
Is that what you needed that lm3886 PCB for?

Good idea to make a test bed btw.
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RDV

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Re: Preamp thoughts
« Reply #8 on: April 05, 2006, 09:04:45 PM »
I also fancy having an all-in-one combo. The little Dean Markley doesn't have room for a speaker in it.

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joecool85

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Re: Preamp thoughts
« Reply #9 on: April 05, 2006, 09:20:32 PM »
I'm thinking about doing a lm1875 combo with 2x8.
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RDV

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Re: Preamp thoughts
« Reply #10 on: April 05, 2006, 09:27:11 PM »
Are you going to build the cabinet? That's one of my weak points. I'm no carpenter.  ::) My sub cabinet came out alright but there are some gaps. Let's just say I didn't have to drill an air hole.

RDV

joecool85

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Re: Preamp thoughts
« Reply #11 on: April 05, 2006, 09:42:34 PM »
Naw, I think I am going to find a gutted cab on eBay and use that.  I'm no carpenter either lol.  I've made solid sub boxes for my car, no leaks or anything, but those are made out of advantec and don't need to be pretty since they are covered in carpet.
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R.G.

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Re: Preamp thoughts
« Reply #12 on: April 06, 2006, 12:19:47 AM »
Quote
I've got to come up with a decent chassis which I may have to actually buy this time.
Get yourself a 2RU rack cabinet. Make the wooden box house a single 12 and the rack cabinet. Place electronics in the rack cabinet.

2U cabs can be had for $50 from Middle Atlantic as I remember.

teemuk

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Re: Preamp thoughts
« Reply #13 on: April 06, 2006, 03:27:25 PM »
What you might be after RDV is not neccessarily a bunch of diodes acting as a limiter but a variable gain or attenuation control. This kind of circuit stops from ever driving a gain stage too hard and therefore it has a much smoother tone - in best circuits it's almost inperceivable. If you want to keep things simple, here's a very simple example of compressing/limiting the signal with a LDR:

http://machines.hyperreal.org/categories/DIY/info/compressor

With an improved detector, circuits like these provide quite good results.

So where should such a circuit exist? Consider this: The easiest way to keep the signal clean is to keep the gain low in every stage. However, this isn't always possible. Whenever thereĀ“s a chance that a stage might be overdriven, there should be a limiter circuit - either within the stage or before it. In most simple form the preamp has a very low gain throughout and the limiter is the last link in the chain before the power amplifier - perhaps located outboard in the effects loop.

It would be good to locate the bunch of diodes we have been discussing about right before the PA input. There they would act as "hard" limiters for the maximum amplitude of the input signal, (which should be the input voltage required for full power aka. input sensitivity). The job of this stage is to clip the transients that the softer limiter is too slow to handle. Why so? Limiting (with, for examle, LDRs) is preventive; meaning you can prevent clipping from ever occuring. However, clipping with diodes is not preventive; therefore the "hard limit" offered by diodes should be the final link in the chain. If you aim for a quite clean tone the hard limit should always be the final resort. Of course you can always use a hard limiter after softer limiting to remove the transients that are caused by the slowness of the detector circuit. Located in this place the hard limiter should be nearly "transparent".

A little expansion beyond preamplifiers: Most of the chip amps, like LM3886, have the benefit that they operate basically like opams - thus feedback/gain is very easy to set and control. Adding a limiter even to this path is therefore quite easy. Here's how Vox did it in the 80's with the compromise of loosing some headroom:

http://www.schematicheaven.com/voxamps/busker.pdf

Quite nice considering that you add only four extra components. The output will clip earlier and the amp will never reach the full power as "clean". However, the clipping has a much more rounder knee. Little less simple, but much more ear-pleasing, way would be to add a LDR somewhere either to control the gain or attenuate the input signal.

R.G. also represented some very good points. I especially liked the one about input protection: Although a typical DIYer probably plans to use the device only by him/herself, (and therefore thinks nothing will ever happen to it), sooner or later there will come a point when someone else get's a chance to hassle with it. Unfortunately, people who are not familiar with basic electronics can do utterly stupid things with good intentions. Another chapter are the hazards caused by pets, little children or simple carelessness. For example, feedbacking guitars have blown numerous amps and speakers. Even "normal" use can ruin some devices: Plugging jacks in and out or leaving the coords that are unplugged from the other end into the amp while the device is on. Even some static charges can do destruction if the device is not protected. A good design practice is to at least consider the possible risks.

RDV

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Re: Preamp thoughts
« Reply #14 on: April 19, 2006, 01:04:07 PM »
That's a very interesting concept by Vox with inserting diodes in the FB loop of that little Busker amp. I wondering though could that somehow be implemented with an LM3886 somehow? I suppose I could try it with my perfboard LM3886 amp easily enough.

Opinions? Ka-Boom? ZZZZzzz sizzle?

RDV